The present invention relates to a method of producing a polyolefin composition using at least two gas phase fluidized bed reactors in co-existence with a saturated aliphatic hydrocarbon, and a method of producing a polyolefin composition wherein a saturated aliphatic hydrocarbon is allowed to exist when transporting a polymer particle from a reactor in the previous stage to a reactor in the subsequent stage.
Polyolefins, for example, linear low density polyethylenes which are a copolymer of ethylene with an xcex1-olefin (hereinafter, referred to as xe2x80x9cLLDPExe2x80x9d in some cases) have various properties such as excellent heat seal properties, flexibility and toughness, excellent water-resistance, moisture-resistance and chemical-resistance, lower cost, and the like, and are widely used conventionally as materials for molding films and as industrial materials, and the like.
LLDPE is usually produced by copolymerizing ethylene with an xcex1-olefin at low pressure in the presence of a transition metal compound catalyst such as a Ziegler catalyst and the like, and as the polymerization method, liquid phase polymerization methods such as solution polymerization, slurry polymerization and the like, and gas phase polymerization methods are used. Production of polyolefins such as LLDPE and the like by a gas phase polymerization method is industrially advantageous since a polymer is obtained in the form of a particle and consequently the production process can be simplified as compared to a liquid polymerization method requiring the steps of precipitating and separating a particle from a polymerization solution, and the like.
Further, LLDPEs having a narrow molecular weight distribution are desired since they can provide films having excellent molding properties and the like.
The present inventors have studied, under such conditions, a method of producing polyolefins such as LLDPE and the like according to a gas phase polymerization method, and resultantly, have found that LLDPE having a narrow molecular weight distribution is obtained at high yield if copolymerization such as ethylene with an xcex1-olefin using at least two gas phase fluidized bed reactors is so conducted that the concentration of a saturated aliphatic hydrocarbon in a reactor is in a specific range and the ratio of the concentration in a reactor of a first stage to the concentration in a reactor of a second stage is in a specific range.
Further, when a polyolefin composition is produced using a multi-stage gas phase polymerization apparatus composed of at least to gas phase fluidized bed reactors as described above, a polymer particle sometimes clogs a transport line for transporting a polymer particle extracted from a reactor in the previous stage to a reactor in the subsequent stage, which obstructs the line.
As the means for solving such a problem, a method is used in which an antistatic agent is added to a transport line, however, catalytic activity in the subsequent stage may decrease, in this method.
The present inventors have investigated, under such a condition, a method of preventing obstruction of a line occurring in producing a polyolefin composition by a multi-stage gas phase polymerization method, and resultantly found that obstruction of a line can be prevented if a specific amount of saturated aliphatic hydrocarbon is allowed to exist in an extraction line and transport line, leading to completion of the present invention.
The method of producing a polyolefin composition of the present invention is a method of producing a polyolefin composition comprising copolymerizing at least two olefins selected from ethylene and xcex1-olefins having 3 to 20 carbon atoms using at east two gas phase fluidized bed reactors in the presence of a transition metal compound catalyst, wherein copolymerization of the reactor in a second or later stage is conducted in the presence of a copolymer produced in the previous stage, and is characterized by that a saturated aliphatic hydrocarbon exists in each reactor in a concentration from 0.1 to 30 mol %, and the ratio of the concentration (C2) of a saturated aliphatic hydrocarbon in a reactor of a second stage to the concentration (C1) of a saturated aliphatic hydrocarbon in a reactor of a first stage (C2/C1) is 0.13 or more.
The method of producing a polyolefin composition of the present invention comprises an embodiment in which a saturated aliphatic hydrocarbon having 2 to 10 carbon atoms is introduced to a fluidized bed of each reactor so that the concentration thereof in a reactor is in the range from 0.1 to 20 mol % and the ratio of a concentration (C2) of the saturated aliphatic hydrocarbon in a reactor of a second stage to a concentration (C1) of the saturated aliphatic hydrocarbon in a reactor of a first stage (C2/C1) is 0.13 or more, and ethylene and an xcex1-olefin having 3 to 20 carbon atoms are copolymerized in the presence of a metallocene-based supporting catalyst, to produce a linear low density polyethylene composition.
According to the present invention, it is preferable to produce a polyethylene of a higher molecular weight having a melt index in the range from 0.01 to 50 g/10 min. and a density in the range from 0.860 to 0.950 g/cm3 in one reactor, and to produce a polyethylene of a lower molecular weight having a melt index in the range from 0.5 to 3000 g/10 min. and a density in the range from 0.900 to 0.975 g/cm3 in another reactor, to give a composition in which the weight ratio of the polyethylene of a higher molecular weight to the polyethylene of a lower molecular weight is in the range from 30:70 to 80:20, and it is also preferable to produce a polyethylene of a higher molecular weight having a melt index in the range from 0.01 to 50 g/10 min. and a density in the range from 0.860 to 0.950 g/cm3in a reactor of a first stage, and to produce a polyethylene of a lower molecular weight having a melt index in the range from 0.5 to 3000 g/10 min. and a density in the range from 0.900 to0.975g/cm3in a reactor of a second stage, to give a composition in which the weight ratio of the polyethylene of a higher molecular weight to the polyethylene of a lower molecular weight is in the range from 30:70 to 80:20.
In the present invention, it is preferable to conduct copolymerLzation at a temperature in the range from the dew point of a mixed gas containing a polymerization monomer and the above-mentioned saturated aliphatic hydrocarbon present in a fluidized bed of a reactor of a first stage to a temperature 5xc2x0 C. higher than the dew point, and it is preferable that the ratio of a concentration C2 of a saturated aliphatic hydrocarbon in a reactor of a second stage to a concentration C1 of a saturated aliphatic hydrocarbon in a reactor of a first stage (C2/C1) is in the range from 0.13 to 20.
Further, in the present invention, it is preferable to conduct copolymerization while controlling the concentration of a saturated aliphatic hydrocarbon in a mixed gas in one reactor to be higher than the concentrations of saturated aliphatic hydrocarbons in mixed gases in reactors of the previous stage.
The method of producing a polyolefin composition according to another embodiment of the present invention is a method of producing a polyolefin composition comprising polymerizing or copolymerizing at least one olefin selected from ethylene and xcex1-olefins having 3 to 20 carbon atoms using at least two gas phase fluidized bed reactors, wherein a saturated aliphatic hydrocarbon is allowed to exist in a transport line for transporting a polymer particle extracted from a fluidized bed reactor in the previous stage to a fluidized bed reactor in the subsequent stage, and the concentration of a saturated aliphatic hydrocarbon in the transport line is in the range from 0.1 to 30 mol %.
In the present invention, it is preferable that all or a part of the above-mentioned saturated aliphatic hydrocarbon is fed to a fluidized bed reactor, and introduced into a transport line together with a polymer particle.